Extrusion apparatuses which comprise a screw rotatable in a hollow cylinder or barrel are known. Such devices include feed apertures for the materials being used. Flow obstruction means are formed on the screw and outlet and inlet apertures disposed, respectively, upstream and downstream of the flow obstruction means are formed in the cylinder or barrel. These two apertures are interconnected through the intermediary of a gear pump. Such an apparatus is disclosed in U.S. Pat. No. 4,431,311, particularly FIG. 2 thereof. In such apparatus, the material leaves the cylinder or barrel through an outlet aperture, which aperture is located upstream of backwash means, constituting the obstruction means, formed on the screw. The aperture conveys the material into a static mixing device through a connection conduit. The material then passes through a gear pump, through a second static mixing device and is then returned into the hollow cylinder or barrel through a suitable located inlet aperture. This ensures through mixing of the constituents of the mixture.
However, there are many disadvantages associated with the provision of such a gear pump. In particular, there are long deflection paths to be covered by the mixture of synthetic material in travelling from the outlet opening from the hollow cylinder through conduit sections to the static mixing means, from the mixing means through conduit sections into the gear pump. These long paths are highly disadvantageous, because deposits are formed in the deflection channels.
On the other hand, it is highly advantageous to provide a gear pump because, by using such a device, a higher, but uniform, build-up of pressure in the material being conveyed can be achieved. This in turn, ensures that the mixture of synthetic material is extruded in a very uniform manner.